It was for his new movie The Five-Year Engagement and the studio head was worried the audience wouldn't believe Jason could get a girl like his co-star Emily Blunt unless he dropped some weight.
That's kinda mean.
But Jason listened and he ended up dropping 35 pounds after working out twice a day with a trainer who also watched what he ate all day.
But Jason had a trick for sneaking food while on set.
Check out the video (above) to hear his sneaky tricks for extra eating.